Axisymmetric, moving coil type electrodynamic loudspeakers generating acoustic waves in response to a current are known. The moving coil borne by a mandrel is integral with a diaphragm, and there exists two main types of loudspeakers according to the implementation of the diaphragm, the cone type and the dome type loudspeakers.
The general operating principle of a loudspeaker is based on the possibility to set in motion a cylindrical coil carrying an electric current and placed in a static magnetic field created by one or more annular or cylindrical fixed permanent magnet(s) which magnetization orientation is parallel to the revolution axis of the loudspeaker, a plurality of ferromagnetic parts channeling the magnetic field so as to bring him radially relative to the coil. The air gap is the place where the coil is located, the coil moves in a free space between faces of internal (toward the central symmetry axis of the loudspeaker) and external (toward the periphery of the loudspeaker) magnetic constructions (generating and/or channeling a magnetic field according to whether they comprise a magnet or not), relative to the mandrel. In the following, it will be referred to internal volume for the free space (comprising the internal part of the air gap) comprised between the mandrel and internal magnetic construction and to external volume for the free space (comprising the external part of the air gap) comprised between the mandrel and external magnetic construction. Magnetic constructions classically implemented in such loudspeakers use ferromagnetic parts to loopback the magnetic field of the magnet(s) in order for it to be able to go through the coil in this air gap. Finally, a loudspeaker comprises a rigid supporting construction called “frame” and enabling the basic components of the loudspeaker to be held in defined static and dynamic structural and functional relations.
In order for the coil to be correctly guided in the air gap, in particular for it not to rub against the edges of the vertical free space/air gap, and to be brought back to a defined rest position in absence of current, it is implemented suspension means of edge type (between the periphery of the diaphragm and the frame) and “spider” type (between the coil-bearing mandrel or the diaphragm and the frame). Besides the double guiding function, these suspension means also fulfill a pneumatic sealing function (in particular the edge) between the faces of the diaphragm, so as to avoid an acoustical short-circuit between the faces of the diaphragm, and a returning function (the edge and the spider) of the coil to a defined rest position.
For more precisions about the construction and the operation of loudspeakers, general explanations and examples about loudspeakers can be found for example in “HIGH PERFORMANCE LOUDSPEAKERS” by Martin Colloms, edited by WILEY, ISBN 0471 97091 3 PPC.